1. Field of the Invention
The present invention relates to a method of determining a semiconductor memory device. More particularly, the present invention relates to a method of determining a native threshold voltage of a nonvolatile memory.
2. Description of Related Art
A typical flash memory has a floating gate and a control gate fabricated using doped polysilicon. When the memory is programmed, suitable programmed voltages are applied to the source region, the drain region, and the control gate respectively. Next, electrons will flow from the source region to the drain region through channel of memory cell. In the process, a portion of the electrons will penetrate a tunneling oxide layer below the polysilicon floating gate layer. Here, the electrons enter and distribute uniformly in the entire polysilicon floating gate layer. The phenomenon of electrons penetrating the tunneling oxide layer to enter the polysilicon floating gate layer is called a tunneling effect. The tunneling effect can be categorized into a channel hot-electron injection and a Fowler-Nordheim tunneling (F-N tunneling). The flash memory is usually programmed with the channel hot-electron injection, and erased with the F-N tunneling by passing the channel region or the side of the source region.
In general, after the flash memory has been manufactured, as each memory cell may be affected by the manufacturing process to result in uneven threshold voltages, the memory will have a larger distribution of the threshold voltages that may cause difficulty in use. Hence, before the shipment, the UV light is usually applied to irradiate the flash memory completely, such that every memory cell of the flash memory is at a stage of low threshold voltage (Low |Vt|) to achieve an initialization of devices. After the memory cell has been irradiated completely by the UV light, the threshold voltage maintained is referred as a native threshold voltage.
However, in the trend of increasing the integration of memory devices, the size of memory cells is also relatively decreased. Moreover, the memory cell is usually covered by high metal density. When the memory is exposed to the UV light, the UV light is blocked by metal layers, so the memory cell will not be irradiated easily. Consequently, the initialization of devices in the memory can not be accomplished. Furthermore, as the UV light can not irradiate the memory cell completely, the memory cell can not reach the stage of the native threshold voltage and the native threshold voltage of the memory cell can not be determined.